System, apparatus &amp; method for arresting propagation of a deflagration in a clean air return duct of an air-material separator

ABSTRACT

Disclosed herein are a system, method, and apparatus for arresting flames in an air return line. The apparatus includes a flame barrier containing one or more metal mesh layers and configured to permit airflow there through while preventing flame break-through. The flame barrier can also have or be connected to one or more temperature or pressure sensors configured to detect blockage of airflow through the flame barrier and to detect damage to the flame barrier. The apparatus can also include additional temperature or pressure sensors for detecting the propagation of deflagration in the air return line.

BACKGROUND

The accumulation of combustible dust during the production of powderbulk solids creates a substantial risk of flash-fire and explosion (a“combustible dust event” or “dust deflagration”). Though the issue ofcombustible dust has been known for a long time, the study andprevention of combustible dust events is often overlooked andunacknowledged. NFPA 652 entitled “Standard on the Fundamentals ofCombustible Dust” is a national standard published by the National FireProtection Association (hereinafter “NFPA 652”). NFPA 652 definescombustible dust as a finely divided combustible particulate solid thatpresents a flash-fire hazard or explosion hazard when suspended in airor the process-specific oxidizing medium over a range of concentrations

Recently, the NFPA revised its standards to require that clean airexhaust lines from air-material separators (dust collectors/filterreceivers/etc.) must include explosion isolation devices unless they aredirected outdoors to a safe location away from people. This newrequirement is creating a challenge for customers since currentexplosion protection equipment for combustible dust is expensive, andinstalling additional isolation devices on process equipment can greatlyincrease the total price of a project.

Although explosion isolation products currently exist for combustibledust applications (including pinch valves, knife gate valves, chemicalblockers and flap valves), they are principally designed forapplications in which material (dust) must flow through the devicesunder normal operating conditions. Accordingly, such devices are complexand expensive, and are generally and economically unsuitable forapplication in clean air return lines. In addition, exhaustingpreviously conditioned (cooled or heated) air to the environment ratherthan returning the conditioned air to a facility results in substantialenergy losses and increases a facility's carbon footprint.

As a result, combustible dust facilities are presently struggling withthe decision whether to add large capital expenses to their budgets toprotect the clean air exhaust lines which return conditioned air backinto the facilities, or to avoid such capital expenses and endure higherenergy costs year after year by exhausting previously conditioned andcleaned air to the atmosphere. Accordingly, there is a need for a simpleand economical device which provides explosion isolation for clean airexhaust lines in combustible dust facilities. Moreover, there is a needfor an explosion isolation device which is reliable, can be applied to awide range of applications, is easy to install, has a low long termoperating cost, and is simple to maintain. In addition, such anexplosion isolation device should be designed to be certifiable underNFPA 69—Standard on Explosion Prevention Systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a dust collection system including aflame arrester according to the invention installed in a clean airreturn line;

FIG. 2 is a perspective view of one embodiment of a flame arrestoraccording to the invention; and

FIG. 3 is a longitudinal sectional view of the flame arrestor shown inFIG. 2.

DETAILED DESCRIPTION

The invention is directed to a device, system and method for preventinga dust deflagration initiated in an air-material separator from enteringa facility through a clean air return line. As shown in FIG. 1, afacility 10 may include a typical air-material separator 12 locatedoutside an exterior wall 14. Alternatively, the air-material separator12 may be located inside from the exterior wall 14 (not shown in FIG.1). A dirty air line 16 directs combustible dust suspended in air(“dirty air”) from the facility 10 to the air-material separator 12.Once the air-material separator 12 separates the dust from the dirtyair, cleaned air is exhausted from the air-material separator 12 and isdirected into the facility through a clean air line 18. This cycle isrepeated such that dirty air from the facility 10 is continually cleanedby the air-material separator 12. A flame arrestor 100 according to theinvention is provided in the clean air line 18. The flame arrestor canbe located at different points in the clean air line 18, includingproximate to the air-material separator 12, proximate to the facility10, or at any point in-between. If a deflagration initiates within theair-material separator 12, the flame arrestor 100 is designed to blockpropagation of the flame through the clean air line 18, and therebyprevent the deflagration from entering the facility 10 through the cleanair line 18.

One embodiment of a flame arrestor 100 according to the invention isshown in FIGS. 2 and 3. In this embodiment, the flame arrestor 100includes a housing 102 having an inlet 105 and an outlet 107. In theembodiment shown in FIGS. 2 and 3, the inlet 105 and the outlet 107 areboth circular and have equal diameters. Alternatively, the inlet 105 andoutlet 107 can have any other shape which is compatible withinstallation in a clean air return line having a particularcross-sectional shape, such as square or rectangular, for example. Thehousing 102 also includes a central portion 113 which is substantiallylarger in diameter than the inlet 105 and the outlet 107. In theembodiment shown, the central portion 113 has a generally squarecross-sectional shape, and has a width which is more than twice thediameter of the inlet 105 and the outlet 107. The central portion 113can have other alternative cross-sectional shapes and sizes. A firsttransition portion 109 of the housing 102 extends between the inlet 105and the central portion 113, and a second transition portion 111 extendsfrom the central portion 113 to the outlet 107. The first transitionportion 109, the central portion 113, and the second transition portion111 combine to form a continuous shell capable of containing anddirecting a flow of air through the housing 102 from the inlet 105 tothe outlet 107. Flanges 117, 119 can be provided on each end of thehousing 102 for use in connecting the flame arrestor 102 to adjoiningsections of a clean air return line.

As shown in FIG. 3, the flame arrestor 100 includes a flame barrier 120disposed within the housing 102. In this embodiment, the flame barrier120 is positioned within the central portion 113 of the housing 102. Inone embodiment, the flame barrier 120 includes a plurality of metal meshlayers 121 stacked together along its depth. Each metal mesh layer 121includes a plurality of openings (not shown) which extend through thelayer 121 and permit air to pass through the layer 121. In oneembodiment, each metal mesh layer 121 is stainless steel, though othertypes of metal also may be used. Alternatively, the metal mesh layers121 may be constructed of two or more various types of metals combinedtogether in a single stack to form the flame barrier 120.

The number of metal mesh layers 121 and the percentage of open area ofthe metal mesh layers 121 can be varied to provide the flame barrier 120with desired properties and capabilities. In particular, the flamebarrier 120 can be configured to permit clean air exhaust to flowthrough the flame arrestor 100 with a minimal pressure drop. In oneembodiment, the flame barrier 120 is configured to provide a pressuredrop from about 0.2 bar to about 0.5 bar. In addition, the number ofmetal mesh layers 121 should be sufficient to quench a flame propagationwhile still allowing air to flow through the flame arrestor 100 duringthe deflagration such that there is no excessive buildup of backpressure in the flame arrestor 100 during such an event. Accordingly,the number and quality of the metal mesh layers 121 is balanced betweena sufficient number of layers to halt propagation of a flame and aminimal number of layers 121 to facilitate air flow. The metal meshlayers 121 form both a choke point for a deflagration and a heat sinkwhich breaks down the deflagration and ceases combustion via thedispersion of heat. The flame barrier 120 is designed to preventflame-break through, and also to prevent any un-burnt dust which mayenter the line during the deflagration from passing through the flamearrestor 100.

The housing 102 is designed to withstand high internal pressuresanticipated during a deflagration, while also maintaining support of theflame barrier 120. In one embodiment, the housing 102 is configured towithstand an internal pressure of at least about 1.0 bar. The housing102 can be constructed of sheet metal or any other suitable material.The housing 102 can include a door or access hatch to permit the flamebarrier 120 to be replaced, if needed, and to permit periodicinspections of the interior of the flame arrestor 100. In addition, aHEPA filter cartridge may be mounted within the housing 102 to eliminatethe need for a separate HEPA filter for the returned clean air, and toact as a silencer for the dust collection system.

The flame arrestor 100 is designed to anticipate potential problems thatend users of the device 100 might encounter. For example, it isdesirable that the flame arrestor 100 is designed such that it is verydifficult or impossible for a typical user of the device 100 to installthe device incorrectly in a clean air return line 18. For example, theflame arrestor 100 shown in FIGS. 2 and 3 may be symmetrical such thatthe device 100 can be installed in opposite directions and still performidentically. In other words, the flame arrestor 100 can be symmetricallydesigned such that the device 100 functions in substantially the sameway when the inlet 105 receives clean exhaust air from the air-materialseparator 12 as when the “outlet” 107 receives clean exhaust air fromthe air-material separator 12. Other methods and configurations may alsobe used to make incorrect or inadequate installation of the flamearrestor 100 difficult or impossible.

In addition, the flame arrestor 100 can include one or more signalingdevices to alert customers of problems and/or to comply with NFPAstandards. For example, in the event that a deflagration does occur, thedevice can be configured to detect the occurrence, and to communicatethe detected occurrence to a user. In one embodiment, the flame arrestor100 includes one or more temperature sensors, one or more pressuresensors, or a combination of one or more temperature sensors and one ormore pressure sensors capable of detecting conditions consistent withthe occurrence of a deflagration event. In addition, the flame arrestor100 can be configured to signal that the flame blocker 120 has beencompromised, and/or that the flame blocker 120 has not been compromised.For example, the flame barrier 120 may include a fusible link that wouldbreak under the heat and pressure of a deflagration and activate asingle pole double throw relay configured to alert a user to an alarmstatus. Furthermore, the flame arrestor 100 can be configured to alert auser that the device has become sufficiently blocked or clogged by dustor other contaminants such that the device 100 may not functionproperly. For example, one or more pressure sensors may be provided todetect any substantial change in the pressure drop across the flamebarrier 120 as clean return air flows through the flame arrestor 100. Inone embodiment, a Magnehelic® differential pressure gauge can be used tomeasure pressure differentials and detect if the flame barrier has beenplugged. The Magnehelic® gauge could be configured to trip a single poledouble throw relay in the event the pressure differential becomes toolarge across the upstream and downstream portions of the arrestor 100.All such signaling devices should be rated for Class 2 Division 2 areasat a minimum, and preferably Class 2 Division 1. These sensors can beconnected to one or more electronic alarm or communication systems whichis/are configured to alert or notify a user when a deflagration event orproblem occurs.

The above descriptions of the invention are intended to discloseparticular aspects and features of various embodiments of the invention.A person of ordinary skill in the art understands that certain changesor modifications can be made to the described embodiments withoutdeparting from the scope of the invention. All such changes andmodifications are intended to be within the scope of this disclosure andany claim appended hereto.

What is claimed is:
 1. A flame arrestor for a return air line from anair-material separator, the flame arrestor comprising: a. a housingcomprising an inlet, an outlet and a central portion, wherein thecentral portion is substantially wider than the inlet and the outlet; b.a flame barrier disposed within the central portion of the housing, theflame barrier comprising a stack of substantially planar metal meshlayers configured to permit a flow of air therethrough; c. one or morefirst sensors for configured to detect a blockage of air flow throughthe flame barrier; d. one or more second sensors configured to detect apropagation of a deflagration in the return air line; and e. one or morethird sensors configured to detect damage to the flame barrier.